A prior art scroll compressor, or pump, 100 is shown in FIG. 8. The pump 100 comprises a pump housing 102 and a drive shaft 104 having an eccentric shaft portion 106. The shaft 104 is driven by a motor 108 and the eccentric shaft portion is connected to an orbiting scroll 110 so that during use rotation of the shaft imparts an orbiting motion to the orbiting scroll relative to a fixed scroll 112 for pumping fluid along a fluid flow path between a pump inlet 114 and pump outlet 116 of the compressor.
The fixed scroll 112 comprises a scroll wall 118 which extends perpendicularly to a generally circular base plate 120. The orbiting scroll 122 comprises a scroll wall 124 which extends perpendicularly to a generally circular base plate 126. The orbiting scroll wall 124 co-operates, or meshes, with the fixed scroll wall 118 during orbiting movement of the orbiting scroll. Relative orbital movement of the scrolls causes a volume of gas to be trapped between the scrolls and pumped from the inlet to the outlet.
A more detailed view of the scroll arrangement is shown in FIG. 9. In the Figure, the fixed scroll 112 is shown in hatching with the scroll plate 120 and the scroll 118, whilst the orbiting scroll is shown in bold with only the scroll wall 124. The scrolls have six successive scroll wraps I, II, III, IV, V, VI between the inlet 128 to the scroll arrangement and the outlet 130. The inlet 128 receives fluid from the pump inlet 114 and the outlet 130 conveys fluid to the pump outlet 116. During relative orbiting motion of the scrolls, fluid conveyed through the inlet 128 is trapped initially in pockets formed in the first wrap I. As the fluid is forced towards the outlet 130 the pockets are gradually compressed through successive wraps II, III, IV, V, VI. The arrangement shown in FIG. 9 is single-start meaning that there is a single generally spiral flow path which starts at the inlet and ends at the outlet.
FIG. 10 shows a double-start, or twin-start, arrangement. As with FIG. 9 the fixed scroll is hatched whereas the orbiting scroll is shown in bold.
Again, the scrolls have six successive scroll wraps I, II, III, IV, V, VI between the inlet 128 and the outlet 130. During relative orbiting motion of the scrolls, fluid conveyed through the inlet 128 is trapped initially in pockets formed in both the first wrap I and the second wrap II thereby forming two fluid flow paths starting at start points 132, 134. This fluid is forced along both flow paths and converges at convergence point 136 forming a single flow path from the convergence point to the outlet 130 through scroll wraps III, IV, V, VI. A multi-start arrangement is typically used when increased pumping capacity is required, that is when it is required that a greater volume of gas is pumped through the pump. Increased pumping capacity is achieved because fluid is pumped directly from the inlet 128 through two wraps I, II rather just a single wrap for a single-start arrangement. However, it will be appreciated that fewer wraps act as compression stages as compared to a single-start arrangement and therefore the ultimate pressure which can be achieved in a multi-start arrangement is less than with a single-start arrangement.
FIG. 11 is graph showing various characteristics of a single-start and a twin-start arrangement when evacuating a chamber initially at atmospheric pressure. The graph shows chamber pressure on the left axis, inverter output power on the right axis and elapsed time on the horizontal axis. Inverter output power is power consumed by the pump. There are four curves shown in the graph; power consumed 138 and chamber pressure 140 for a single start arrangement and power consumed 142 and chamber pressure 144 for a two-start arrangement. Power consumed is shown in broken lines and chamber pressure is shown in solid lines.
Looking first at the chamber pressure plots 140, 144 it will be seen as indicated above that after an initial pressure decrease to 100 mbar, which both single-start and two-start achieve at a similar rate, the two-start arrangement reduces pressure at a faster rate than the single-start arrangement. However, the single-start arrangement produces a lower ultimate pressure (0.005 mbar) than the ultimate pressure achieved by the two-start arrangement (0.01 mbar).
The power 142 consumed by the two-start arrangement is greater than that the power 138 consumed by the single-start arrangement over the initial period from 1000 mbar to 100 mbar, but subsequently the power consumed by the two-start arrangement is less than that consumed by the single-start arrangement.
Depending on the particular pressure regime required in a chamber evacuated by a vacuum pump, a pump with an appropriate configuration is selected. For example, if a low ultimate pressure is the most important characteristic, a single-start pump is used or if rate of pressure reduction is the most important characteristic a two-start pump is used.
Typically, the power consumption of a pump is reduced by limiting the inlet capacity or avoiding high compression ratios. A pressure relief valve is sometimes used in a two-start pump to reduce power consumption.